Simplify `get_restart_data` for GW values

[ccarouge]

The GW values were correctly added to the restart file, however, the code to read the values from the restart also set default values if the variable is not found. We do not want this behaviour. The GW variables should be treated like the other ones:

• set default variables in get_default_inits() (I haven't check if it's the correct place, only going from function name and vague memory).
• replace the default by what is read from the restart file if found. Nothing happens if the variable is not in the restart file.